1. Field of the Invention
The present invention relates to methods for modifying oxidizable surfaces, including diamond surfaces, including methods for metallizing these surfaces, where these methods include plasma oxidation of these surfaces. The present invention also relates to the products of these methods.
2. Description of the Related Art
Modified surfaces are increasingly of great technological importance. In particular, modified diamond surfaces have increasing technological importance because of the attractive physical properties of diamond, including hardness, broad wavelength transparency, high thermal conductivity and sound propagation velocity, variable electrical resistivity, low friction coefficient, high refractive index, and chemical and biological inertness. Applications for modified diamond surfaces exist or are foreseen in a number of areas, including microelectronics (semiconductive layers, optical and x-ray masks, and heat dissipators), optics (transparent or antireflective coatings), machine tools (abrasion resistant coatings), and biotechnology (biocompatible coatings for implants, biosensors).
Of particular interest is the deposition of adherent, conductive metal coatings (including metal contacts) onto these surfaces for microelectronic and mechanical applications. For instance, metal-coated dielectric, such as polymers and ceramics (including diamond) have increasing technological importance in a variety of applications. These applications include use in circuit boards, use for shielding, lithographic masks, etc.
Surfaces coated with various chemical functional groups also are of increasing technological importance. Chemical sensing, including biosensing, is just one area where surfaces coated with chemical functional groups may be employed. For instance, a surface is coated with a particular antigen may be used in testing for a particular antibody.
For many of these applications, it is desirable for these surfaces to be modified in a pattern. The terms xe2x80x9cpatternxe2x80x9d and xe2x80x9cpatterningxe2x80x9d have meanings that vary from application to application. However, for each area of application, these terms are well-understood by skilled practitioners in the art. For example, in the context of fabricating circuit boards, patterned metallization means laying down conductive pathways on a circuit board, preferably with through-holes and other useful structures. In the context of microelectronic applications, patterned metallization means laying down conductive pathways with linewidths in the sub-0.5 xcexcm range, consistent with VLSI applications. Preferably, in the context of microelectronics, these linewidths are about 0.1 xcexcm, using currently available lithographic techniques. As x-ray lithographic techniques improve, it is anticipated that the present invention will produce microelectronic circuits with linewiths of about 0.05 xcexcm. In the context of lithography, patterning means creating a pattern of lines on a mask with sufficient resolution and packing density for the particular application at hand. In the context of chemical applications (such as chemical sensing) patterned chemical modification means attaching chemical groups in a pattern consistent with the specific application and system at hand.
A problem associated with patterned metallization is that it is typically accomplished by means of a xe2x80x9clift-offxe2x80x9d process, in which metal is evaporated or sputtered over a lithographically defined photoresist, and then dissolution of the remaining resist removes metal from selected regions of the metallized surface. This process, which is subtractive in nature, wastes metal and is more difficult and expensive than a process in which metal is deposited in an additive fashion, i.e. only in regions where it is required.
Various workers have fabricated contacts and metal patterns on diamond using sputter deposition or evaporation of Ag, Cu and Au, Al, W and numerous alloys. The principal problems with forming diamond/metal interfaces fabricated with conventional methods are obtaining reproducible and controllable electrical characteristics and obtaining satisfactory adhesion of the metal to the diamond surface.
Acceptable adhesion of metal to diamond and good ohmic contact can be obtained if the metallized substrate is annealed at high temperature (800-900xc2x0 C.), forming a metal carbide layer at the interface. However, such high temperature is also problematic for metal/diamond contact formation because the diamond surface is subject to graphitization, which can degrade the electrical properties of the interface. Vacuum metallization is another available method for metallizing diamond surfaces, but this method requires complex, expensive equipment.
A problem with forming polymer/metal interfaces is that currently employed techniques for surface modification result in a surface that is roughened on the microns scale. For many applications, especially microwave and millimeter wave applications, a much smoother surface is desired.
The principal problem encountered in modifying diamond and other surfaces, both for metallization and other types of functionalization, is putting a sufficient density of functional groups on the diamond surface to carry out a chemisorption reaction. As is shown below in the summary of the invention, plasma oxidation processes are useful for this purpose. Other processes may also be useful. Diamond surfaces may be modified by detergents and acids, which produce oxygen-containing functionalities on the diamond surface. However, these processes may not result in a sufficient concentration of the type of oxygen-containing functional groups needed to carry out a commercially viable chemisorption reaction. Only oxidation processes that result in high concentrations of oxygen-containing functional groups that participate in chemisorption reactions are useful for modifying oxidizable surfaces.
Adhesive metal patterns can be deposited selectively on a wide variety of surfaces using ultrathin film (UTF)-based metallization processes. See co-pending application Ser. No. 07/691,565; see also Schnur et al., U.S. Pat. No. 5,077,085, issued Dec. 31, 1991; Schnur et al., U.S. Pat. No. 5,079,600, issued Jan. 7, 1992; Calvert et al., New Surface Imaging Techniques for Sub-0.5 Micrometer Optical Lithography, Solid State Technology 34, 77 (October 1991); Dressick et al., Selective Electroless Metallization of Patterned Ligand Surfaces, Proc. Materials Research Soc""y 1992 Spring Meeting Symposium C: Advanced Metallization and Processing for Semiconductor Devices (in press); Calvert et al., Top Surface Imaging Using Selective Electroless Metallization of Patterned Monolayer Films, ACS Symposium Series on Polymers for Microelectronics (invited paper), which are incorporated by reference herein.
These references describe methods and materials for the attachment (by chemisorption) of ultrathin film (UTF) precursor materials (such as organosilanes or organotitanates) to surfaces that either intrinsically possess, or are treated to have, highly reactive polar surface groups. The references also teach patterning the chemisorbed UTF layers with actinic radiation, catalyzing the patterned UTF layers by exposure to either a Pd/Sn colloidal catalyst or a tin-free Pd catalyst, then selectively metallizing by immersion in an electroless plating bath. Omission of the irradiation step leads to a homogeneously metallized, rather than a patterned, surface.
Many surfaces, however, do not intrinsically possess a suitable density of reactive surface groups to carry out a commercially viable chemisorption reaction to attach UTFs. These surfaces include diamond and polymers such as polyethylene (PE) and polyethersulfone (PES). Direct treatment of these substrates with UTF precursors does not lead to a useful degree of surface functionalization. To modify or metallize these surfaces, a different approach is needed.
These surfaces can be rendered amenable to chemisorption reactions by carefully chosen surface oxidation pretreatment. Many methods for oxidizing surfaces are known, some of which include treatment with KMnO4, chromic acid, molecular or atomic oxygen, ozone, peroxide, persulfate, perchlorate, etc. The nature, distribution, and density of oxidized surface functional groups depends greatly on the type of oxidant, the substrate, and the particular treatment conditions. Thus, although a variety of oxidation pretreatments are available, few are actually useful for practical applications due to factors such as reagent toxicity, environmental unsuitability, need for expensive capital equipment, undesired side reactions, unsuitable reaction conditions (high temperature, long treatment times), or inability to produce the specific type and density of oxidized surface functional groups required for UTF chemisorption. In the co-pending application, the PE and PES surfaces were oxidized in a hot acidic potassium dichromate solution for 90 minutes to obtain surface functionalization with ligating UTFs and subsequent electroless metallization. This approach is not particularly desirable because of the use of highly corrosive, toxic and environmentally problematic chemicals, as well as long treatment times.
Relatively little is known about the effects of varying oxidation conditions on diamond surfaces. Known methods for oxidizing diamond include heating in O2 and O, and treatment with detergents and acids. For several reasons, these processes are undesirable as oxidative pretreatments for UTF surface functionalization. Oxidation in O2 at elevated temperatures is highly complex and, depending on conditions, may be slow and may lead to graphitization, faceting and etching. The diamond surface oxygen-containing groups generated by exposure to O2 or O are thought to be primarily bridging etheric and carbonyl oxygen groups. These surface groups are not readily amenable to chemisorption. The nature and distribution of oxidized surface functionalities formed by oxidation in detergents and acids are not known.
Because of the uncertainty associated with the nature of the untreated diamond surface, it was necessary to develop procedures for diamond oxidation that would lead to sufficient functionalization for the selective metallization process to occur. A practical, reproducible method is needed for oxidizing surfaces as a pretreatment for carrying out a chemisorption reaction, where this oxidation method forms significant densities of groups that are readily amenable to chemisorption.
Another problem is encountered in modifying many polymer substrates: swelling, blistering and dissolution of the polymer substrate when the chemisorption reaction is carried out in organic solution. Therefore, it is desirable to carry out the chemisorption reactions in substantially aqueous solutions (i.e. solutions without sufficient quantities organic solvents to cause these deleterious effects). Aqueous solutions have the additional advantages of being more economical, environmentally friendlier, and safer than many organic solutions.
Accordingly, it is an object of this invention to oxidize surfaces, including diamond surfaces, to put a sufficient type and density of oxygen-containing functional groups on the surface to perform commercially viable chemisorption reactions.
It is a further object of this invention to perform chemisorption reactions on these surfaces, thereby attaching a homogeneous layer of chemical functional groups to these surfaces, where these layers optionally form patterns.
It is a further object of this invention to perform these chemisorption reactions in substantially aqueous solutions.
It is a further object of this invention to deposit on these surfaces, adherent, conductive metal coatings with reproducible and controllable characteristics, at temperatures well below annealing temperatures, without roughening these surfaces by an amount that will significantly affect the transmission of microwaves or millimeter waves, and to deposit these metal coatings additively, and without the use of highly toxic reagents such as chromic acid, and where these metal coatings optionally form patterns.
These and other objects of the invention are accomplished by the structures and processes hereinafter described.
The method of the present invention comprises oxidizing a surface in an oxygen-containing plasma, and attaching to the oxidized surface chemical functional groups capable of bonding to the oxidized surface by performing a chemisorption reaction between the oxidized surface and the chemical functional groups. The method of the present invention may optionally include the additional step of metallizing the functionalized surface by immersion in an electroless plating bath. The method of the present invention may optionally include the additional step of photolithographically patterning the functionalized surface with actinic radiation. If the surface is metallized, any photolithographic patterning step is carried out prior to electroless metallization. The process of the invention for selectively metallizing diamond films is described in Calvert et al., Selective Metallization of CVD Diamond Film, Proc. Materials Research Soc""y 1992 Spring Meeting, Symposium C: Advanced Metallization and Processing for Semiconductor Devices (in press), which is incorporated by reference herein.
The product of this method is a modified surface, which optionally may be selectively patterned. This surface may be a diamond surface or some other oxidizable surface. If the surface is diamond, it may be free-standing or disposed on a substrate. The modified surface may be metallized or chemically functionalized. The selectively patterned metallized surface may form a printed circuit board, and may include through-holes and other useful structures.
The functional groups referred to in this application are only those functional groups that react with surface oxygen groups in a chemisorption reaction. Furthermore, in the context of metallization, the functional groups referred to in this application are only those functional groups which are catalyzable for electroless metallization.